Receive the latest national-international updates in your inbox

This Oct. 21, 2017, file photo shows members of the 3rd Special Forces Group (Airborne) 2nd battalion Fse salute the casket of U.S. Army Sgt. La David Johnson at his burial service in the Memorial Gardens East cemetery in Hollywood, Florida. Sgt. Johnson and three other American soldiers were killed in an ambush in Niger on Oct. 4.

The U.S. special forces unit that came under attack in Niger earlier this month had been pursuing a senior militant who escaped, multiple U.S. officials told NBC News.

The officials did not provide the name of the target, whom one of the officials described as an ISIS recruiter.

Marine Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, told reporters Monday that the Green Berets set out on a reconnaissance mission, and that the intelligence suggested there was a low risk of contact with the enemy. He also said the military was investigating whether the mission changed as it unfolded.

One theory, said an official with direct knowledge of the military's investigation, is that the soldiers were gathering information about the target, and, after learning his whereabouts, decided to pursue him. A big question would then be whether the unit got authorization, and whether the risks were assessed.

Questions Remain for Trump Administration on Niger Mission

In Washington, there is a search for answers about the ambush in Niger that killed four U.S. Service members.